Thermal batteries are a well-known type of primary reserve battery that have a long shelf life at ordinary temperatures and are actuated by heating to a temperature that melts a salt electrolyte. The batteries are useful in applications for a high energy output over a short time. The batteries comprise a plurality of cells, each of which includes an anode, a cathode, an electrolyte and a heat source, usually an ignitable exothemically reactive chemical charge or pyrotechnic. A variety of electrochemical systems are known for use in thermal cells.
The anode is an electropositive metal, commonly calcium, lithium alloys solid at the battery operating temperature or molten lithium as described in U.S. Pat. No. 3,930,888, U.S. Pat. No. 3,891,460, U.S. Pat. No. 4,221,849, and the application of Clinton S. Winchester, Ser. No. 554,005, filed Nov. 23, 1983, of common ownership with this application. The electrochemical reactant of the cathodes, referred to as depolarizers, are electronegative materials that are reduced in the electrochemical cell reaction and include phospates, borates, metal oxides and chromates. U.S. Pat. No. 4,119,769 discloses the use of an iron pyrite (FeS.sub.2) depolarizer. Under current practice powdered iron pyrite is mixed with electrolyte, and sometimes binder, and compacted to form a homogeneous wafer cathode, which may be a catholyte layer of a composite wafer also having an anolyte layer comprising electrolyte and binder. The electrolytes are ordinarily mixtures of alkali metal halides, most often a eutectic mixture of lithium chloride and potassium chloride.
It has been known that thermal batteries having an iron pyrite depolarizer exhibit an initial peak voltage transient (spike) that has been attributed to the presence of free sulfur in the pyrite (Dand and Press, Improved Thermal Battery, AFAPL-TR-79-2027, April 1979) or surface impurities resulting from the reaction of FeS.sub.2 and atmospheric oxygen (Levy and Craftsm Studies of the Abnormally High Peak Voltage Observed with FeS.sub.2 Depolarized Thermal Batteries, SAND 79-0090, Sandia Laboratories, April 1979). In some applications the voltage transient is undesirable and additive CaSi.sub.2 has been added to the depolarizer to suppress it (U.S. Pat. No. 4,383,014).
In some systems, however, high voltages are used to activate electronic circuitry, and voltage transients are desirable. The voltage transients using iron pyrite depolarizers are not sufficiently reliable and reproduceable for such purposes, the transients are variable from lot to lot of pyrite, and iron pyrite alone may not give a sufficiently high voltage spike of the necessary duration.